(1) Field of the Invention
This invention relates to a screw with a thin head which does not uglify a work piece even though the screw head juts from the work piece when it has been screwed into the work piece, and can work piece without causing any problem with its fastening function.
(2) Description of the Related Art
Recently, electric appliances and intelligence apparatus have been improved in performance and comparatively small-sized ones have been used. Especially, thin electronic apparatus such as electronic pocketbooks, notebook-sized personal computers, and handy phones have been rapidly widespread because of their portability. These apparatus must be as light in weight as possible and be thin to ensure their carryability. As a natural result, small screws whose major diameters of external threads are 2 mm or less have been used to assemble such apparatus. Therefore, smaller screws have been developed in recent years.
Besides, the screws have been in demand with thinner heads in recent years. This type of screws are invented and then announced as Utility Model No. 38-10834 in 1963 as shown in FIG. 7. This screw consists of the round head (302) and the main rod (304) with which the round head is integrated. The plain rim (305) is formed around the round head (302); the cross-shaped recess (310) is formed in the round head (302) to fit with a driver; the conic part (not shown in the drawing) is formed at the end of the main rod (304); and the ridge of the same pitch (303) continuously run through the main rod (304) and the conic part. This screw was originally designed not to assemble comparatively thin apparatus, but to integrate a screw head with a washer. The screw was far from the user""s demand in assembling thin apparatus because the whole head was still large. And also in this type of screws an edge is formed between the unthreaded part and the thread portion, because there is an unthreaded part between the head and the thread portion and the thread portion is directly connected to the unthreaded part. The stress concentration at the edge may cause rupture of the screw at time of fastening it and easily lead to a fatigue fracture of the screw due to fatigue building in the edge with time. The flowability of material cannot be attained when a screw head is cast, because the gradient of the tapered unthreaded part is not larger than the gradient of the external wall of cross-shaped recess. Therefore, there is a problem that the life-spans of a punch and a holder used to form screws are short.
Although thin apparatus is in demand, screws compliant with such downsized apparatus have not been developed yet, because screws have a limitation in downsizing. There is a concern that this limitation will wield a very large influence over assemblies of downsized apparatus in the future. To dispel the concern, a screw with a head thinner than those of conventional screws whose major diameter of external threads are just designed to be small has been developed and experimentally used. In heading a screw with a thin head, the slot fitting with a driver bit reaches the shank, and therefore the material around the slot becomes thin and liable to be broken, not ensure the sufficient strength and function of the screw. When a screw is heat treated and plated to be strengthened, the screw heads come into contact with others during the processing in the course of mass production because the volumes of the flat heads are small, causing an incompleteness of a heat treatment and plating.
In keeping with the principles of the present invention, the object of this invention is accomplished by a unique structure comprising a screw head (2) in which a recess (10) coinciding with a driver bit that transfers driving force is formed and a shank (4) which is integrated with and a thread portion (3) is formed on, featuring a circular arc of a comparatively large radius formed on the head top (5); an unthreaded part (20) with larger diameter at a bottom of the head and smaller diameter at an upper end of the thread; which is tapered down between a bearing surface (6) and the threaded portion (3) of the shank (4); a radius (21) with a curved surface at a sectional view along almost one pitch of thread, which is formed at an end of the unthreaded part (20) of the shank in such a way as to be directly connected to the thread portion (3) preventing stress concentration from occurring herein at and after the fastening of the screw.